1
Presented at the 2008 ACEEE Summer Study on Energy Efficiency in Buildings, Pacific
Grove CA.
Appliance Energy Use in America’s Second Home – The Automobile
Valerie M. Thomas, Georgia Institute of Technology
Alan K. Meier, Thomas P. Wenzel, Lawrence Berkeley National Laboratory
Siva G. Gunda, University of California, Davis
ABSTRACT
The average American spends nearly 500 hours per year in motor vehicles. This
exceeds the time they spend in all buildings except for their primary residence. The fuel
consumption of motor vehicles while moving is reasonably well captured in the fuel
economy tests; however, most of the “domestic” appliances—the lights, heater, air
conditioning, entertainment, etc. - are switched off during the test and are poorly
documented. The vehicle versions of some appliances – notably the air conditioning and
audio equipment—are comparable in size and usage of their counterparts in stationary
homes.
Building-like energy use in vehicles parallels that found in homes, but with some
notable differences. The electricity use from appliances such as DVD players, laptops,
and vehicle refrigerators remains modest, although stand-by power use is a growing
issue. Vehicle air conditioners are much less efficient than residential air conditioners,
with the result that vehicle air conditioners in the U.S. consume a total of about 0.9
Quads, comparable to the 2.3 Quads used by air conditioners in residences. Vehicle
heating, in contrast, is a model of efficiency, running as a combined-heat and power
system using waste heat from the motor.
The efficiency and cost of vehicle-generated power depends on the appliance and
how and when it is used, but even in the best case the efficiency is lower and the cost is
higher than for typical residential electricity services. Generation efficiency ranges from
10 to 25%, and for gasoline costs of $3.20 per gallon, the cost of power ranges from
37¢/kWh to $1/kWh.
Vehicles as a Type of Building
Americans spend an increasing number of hours each day in automobiles. To be
sure, the principal reason is to move from one location to another but the driver and other
occupants also accomplish other tasks while in their vehicles. Many of these activities
are familiar to buildings researchers: drink, eat, listen to audio entertainment, shave,
communicate with friends on the telephone, and watch videos. All of these activities
traditionally occurred inside buildings and have either shifted or expanded to mobile use.
Furthermore, appliances that once resided solely in buildings are now becoming common
in vehicles or being moved from one to the other.
These trends suggest that the automobile has become an extension of the
residence and office. It is therefore important to understand the energy implications of the
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buildings-like activities within cars and their relationship to other buildings. Are there
energy-saving opportunities in motor vehicles related to these new loads? Alternatively,
can energy-saving strategies in vehicles be transferred to “stationary” buildings? The
goal of this paper is introduce and describe a new building type and describe buildingsrelated energy use in motor vehicles.
This paper is limited to an analysis of building-like energy use in light-duty
vehicles. However, a similar situation exists with heavy-duty long-haul tractor-trailer
trucks. Truck drivers often pull off the side of the road or into truck stops to sleep after
their driving shift, and many run their engines through the night to provide heating,
cooling, and electricity for other uses such as lighting or entertainment. This practice has
lead EPA and several states to provide incentives for truck stops to provide electricity
“hookups” to power these end-uses when the truck is not moving.
Vehicle Occupancy Levels
Estimates for the amount of time the average person spends in a vehicle range
from 41 (Fischer and Sand 1997) to 95 minutes (Klepeis et al. 2001) per day. All
respondents to the 2001 National Household Travel Survey (NHTS) spent an average of
66 minutes per day in a vehicle, whether it was moving or not. The national Human
Activity Pattern Survey (HAPS) indicates that the average American spent 79 minutes
per day in an enclosed vehicle in 1992-94. Both of these estimates are per survey
respondent, including people that were not in a vehicle at all during the day in question
(Klepeis et al. 2001).
The HAPS also recorded what other environments respondents occupied during
the survey days. Table 1 summarizes the results of the HAPS, for all respondents and for
those respondents that actually occupied the environment in question (called “doers”).
For example, the average time all respondents occupied a vehicle was 79 minutes per day
(481 hours per year). However, only 83% of respondents actually occupied a vehicle
during the survey; this subset of respondents was in a vehicle an average of 95 minutes
per day (578 hours per year). (The average American spent only 78 minutes per day in
an office/factory, because only 21% of those surveyed visited an office/factory during the
survey; those that did visit an office/factory spent an average of 388 minutes (6.5 hours)
per day there.) Leech et al. (2002) report that Americans spend 5.7% of their time in
vehicles and 3.9% in public schools.
Table 1. Estimates of Time Spent in Specific Environments
Mean time
(min/day) overall
Percent of average day
spent in each
environment overall
Residence
990
69%
Mean time
(min/day)
for “doers”
996
Office/factory
78
5%
388
Bar/restaurant
27
2%
112
Other indoor
158
11%
267
Enclosed vehicle
79
5%
95
Outdoors
109
8%
184
1,441
100%
N/A
Environment
Total
3
Source: Klepeis et al. 2001.
From Table 1, the total number of hours spent in vehicles can be calculated as
either 79 minutes/day times total population (296.4 million in 2005), or 95 minutes/day
times vehicle population (238.7 million light-duty vehicles in 2005). These figures give a
range of 138 to 142 billion total hours spent in vehicles in 2005; we use an estimate of
140 billion person-hours per year.
According to national transportation surveys, the amount of time the average
American spends traveling each day has increased nearly 2 minutes every year, from 46
minutes a day in the 1983 survey to 79 minutes a day in the 2001 survey (Polzin et al.
2004).
What Buildings-Related Energy Consumption Occurs in Vehicles?
The car has a building envelope, with walls, windows—lots of them— doors, and
air infiltration. These elements have R-values, transparencies, and air leakage which, if
not controlled will create thermally uncomfortable conditions. Like a building, a car is
heated, cooled, and ventilated. The modern car offers its occupants illumination, a range
of consumer electronics, and food-related services. 1 These are all familiar end uses in
buildings. The energy devoted to propelling the car is not considered buildings-related
(and still represents the overwhelming majority of energy consumption of vehicles). It is
relatively easy to distinguish between the motion-related and buildings-related. In our
compilation, only headlights could be reasonably assigned to either category. 2 There are
also increasing electric loads from vehicle-related equipment including anti-theft
equipment, tele de-lock, and electronic engine controllers which are kept in wake mode
for some time after stand-still to provide quick re-start (Meissner & Richter 2003).
Non-Electrical Building Services in Vehicles: Heating and Cooling
Automotive heating systems use waste heat from the engine and, except for the
fans, are not electrically powered. Cooling systems in light-duty vehicles run about 80%
on shaft power and 20% on electricity produced from the alternator.
Air conditioning in light-duty vehicles consumes roughly 5.5% of all vehicle fuel
(Rugh et al. 2007), corresponding to 0.94 Quads of primary energy per year (Davis &
Diegel 2007). For comparison, air conditioning in the residential sector is responsible for
2.3 Quads (DOE 2006).
Air conditioners in cars have unusual design constraints. They must be sized in
order to quickly lower the temperature of a dangerously hot (over 150 degrees F) vehicle
that has sat in the sun for a long period down to a comfortable temperature (Rugh et al.
2007). The cooling capacity is typically around 6 kW (20 kBtu/hour), which is similar to
that needed to cool a house. Thus, for most situations, the air conditioner is drastically
1
These services are provided by both combustion and electrical devices though, like buildings, a
continuing trend towards electrification is occurring. Renewable energy technologies—in the form of small
PV arrays—are now appearing.
2
We assigned headlights to buildings because they represent a form of “outside lighting”, which is
typically included in residential energy use. Turn signals, on the other hand, are a mobile use.
4
oversized. Conventional auto air conditioners cannot modulate output so the temperature
is adjusted by mixing heated air from the engine, similar to terminal re-heat in
commercial building HVAC systems. The typical system COP for an auto air conditioner
is about 1.6 (EER = 5.5) (Gado 2006) but test conditions differ from residential units so
these values are only indicative (Hendricks 2003).
Strategies to reduce vehicle air conditioning fuel use include variable output
compressors, reduced thermal loads using reflective paints and glazings, better controls,
and consumer information (Rugh, Hovland, & Andersen 2004). There is also a trend
towards electrically-powered air conditioners, especially in hybrid vehicles (Meissner &
Richter 2005). This approach allows variable output and operation while the engine is
switched off. Rooftop photovoltaic collectors have also been investigated as a means of
cooling vehicle cabins while the car is parked (John Rugh et al. 2007).
NREL has estimated that, on average, vehicle air conditioning is switched on 34%
of the time, ranging from 6% of the time in Alaska to 69% of the time in Hawaii
(NESCCAF 2004; Johnson 2002). A survey of 10 instrumented vehicles in Southern
California found that vehicle air conditioners were used in 69% of trips, and that, when
air conditioners were switched on, the compressor was on an average of 64% of the
duration of the trip (Levine et al. 2000).
A transition toward electric vehicles will increase the reliance of vehicles on
electricity for heating and cooling.
Electrically Powered Building-Like Services in Vehicles
Lighting
Lighting energy consists of interior and exterior lighting. We treat headlights as
part of buildings-related energy consumption. The IEA estimated the electricity
consumed by each external light function based on the power and number of hours of
operations (IEA 2006). The data are summarized in Table 2.
In 1995, the typical car had four interior lights. In 2001 the average had already
climbed to 15 (Murphy 2001). Some are rated at above 10 W (though a shift to LEDs is
underway). We assumed eight 3-watt bulbs in the average vehicle and that lights were
operated for 3 minutes/day, which might occur as a result of door opening and closing
(see Table 2). However, some lights may actually stay on while driving or occupants
may deliberately keep some lights on while the car is stationary; including these activities
could easily double energy consumption.
There are also an increasing number of products which may either come with the
car (GPS navigation systems) or may be purchased by consumers for use in the car (seat
warmers, refrigerators for vehicles), or are home appliances that are also used in vehicles
(laptop computers). Power inverters , offered as original equipment or as retrofits, can
provide up to 150 or 350 watts of AC power and can be used to provide power to run a
TV, DVD, video game system, or other products. Cigarette lighter power adapters are
now being marketed for running computers, TVs, DVDs and other equipment for hours at
a time when the car is off (SterlingTek 2008). Table 3 summarizes typical power
requirements for energy-using products designed for vehicles.
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Table 2. Lighting Use in Light-Duty Vehicles
Light
Headlamps
(high beam)
24
65
1.6
Est. percent
time in
operation
(based on 480580 hrs
driving/year)
4-5%
Headlamps
(low beam)
115
55
6.3
20-24%
Rear tail lamps
115
7
0.8
20-24%
Daytime
running lamps
141
40
5.6
24-29%
Interior lights
146
20
2.9
25-30%
5
?
5
?
Brake lamps,
turn signals
Dashboard
lights
Total
Operating
Time
(hours/year)
Typical Power
for
Incandescent
Design (W)
Annual
Energy Use
(kWh/year)
17.3
Source: IEA 2006
Neither the number of vehicles with these appliances, nor the usage of these
appliances are known. To illustrate the potential energy use of these appliances, we
illustrate the energy use of these appliances if they were used 100 hours per year. 100
hours corresponds to using these appliances about 20% of the time that an average
vehicle is driven. It is plausible that some appliances in Table 3 might be used more than
100 hours per year – audio systems, perhaps – and some appliance might be used less –
seat warmers perhaps.
Table 3. Electric Appliances Used in Vehicles
Appliance
Illustrative
Typical Power
Illustrative
Operating Time
(W)
Annual Energy
(h/yr)
Use (kWh/yr)
Large speakers/stereo
100
200
20
TV/DVD combo
100
150
15
GPS Navigator
100
88
8.8
Seat warmers
100
40
4
Moderate speakers/stereo
100
60
6
Refrigerators
100
60
6
Laptop computer
100
50
5
DVD
100
50
5
13” color TV
100
50
5
Car stereo systems (simple)
100
40
4
Video Game Console
100
20
2
Charger for cell phone/PDA
100
15
1.5
Standard car radio
100
4
0.4
Sources: Audio: Crutchfield 2007; Z28 Forum 2007; Mikhaylova 2003. GPS: Al-eds, 2008; Refrigerators:
PPL 2008; Seat warmers: Smarthome, 2008; Other: JC Whitney 2008.
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Overall power used in vehicles is reported to have risen from less than 500 W in
the 1960s to more than 2kW by the year 2000 (Meissner & Richter 2003). The net
power use is much less than in typical homes; however (as noted below), the costs of
operating these appliances is inflated by the high cost of vehicle-generated electricity.
Energy Needed to Produce a kWh in a Vehicle
The energy needed to produce a kWh in a vehicle varies depending on the
appliance and how and when it is used. Figure 1 shows power generation modes in
vehicles and their corresponding efficiencies. The conversion of the chemical energy of
the gasoline into mechanical energy at the crankshaft is about 25-30% efficient (DOE and
EPA 2007; Fenske et al. 2006). The belt that transfers power from the shaft to all
mechanical loads is about 95% efficient (Gates 2008), which means that mechanical
loads have an efficiency of about 26% (23.8-28.5%).
Figure 1. Power generation pathways and efficiencies
Net pathway
efficiency
Power generation and usage steps
~ 23.8-28.5%
Engine
efficiency to
convert
chemical energy
in fuel to
mechanical
energy at the
crankshaft ~ 2530 %
When the
engine is
running, the
battery acts as a
stabilizer.
DC-AC converter
Therefore, there
for plug loads
are no
have an
significant
efficiency of ~
charging75-90%
discharging
losses
Belt efficiency
Alternator
that transfers
efficiency that
power from
converts
shaft to all
Wiring
mechanical
mechanical
connections
loads, typically energy from the efficiencies are
When engine is
serpentine belt connecting belts
~ 100%
to electrical
turned off, all
~ 95%
energy is ~ 50electrical
62%
components run
on the battery
power and the DC-AC converter
for plug loads
associated
have an
chargingefficiency of ~
discharging
75-90%
efficiencies are
in the order of ~
70-92%
~ 11.8-17.8%
~ 8.9-15.9%
Load
~ 8.3-16.3%
~ 6.2-14.6%
Mechanical energy is converted to electricity with an alternator; alternators have
efficiencies in the range of 50-62% (Bosch 1996). The wiring has negligible losses.
When the engine is running, the battery acts as a stabilizer, so there are not significant
charging/discharging losses and electricity production therefore has an efficiency of
about 15% (11.8-17.8). AC devices that are plugged in to the cigarette lighter or other
plug will incur an additional loss from the conversion of DC to AC at an efficiency of 75
to 90% (All-battery.com 2008; Labkorea.com 2008), resulting in plug load efficiency of
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about 12% (8.9-15.9%) when the vehicle is turned on.
When the vehicle is turned off, all electrical components run on battery power; the
charging/discharging efficiency is in the range of 70-92% (DOE 1995), so the efficiency
of electricity production is only about 12% (8.3-16.3%) and only about 10% (6.2-14.6%)
for plug loads.
Table 4. Energy efficiency and power cost for vehicle appliances
Power
generation
Costs ($3.2/gal)
& (36.63
kWh(th)/ gal) =
8.74
cents/kWh(th)
Average cost
A/C unit,
Roughly 80-20
Case 1: Part of compressors run
shaft to direct
the appliance directly from the
alternator load
shaft through a
runs on shaft
distribution,
power and rest belt and blower
equates to ~
and the controls
on electrical
21.4-26.4%
run of the
power
efficiency
electricity.
$0.33 - $0.41
/kWh
$0.37 / kWh
Case 2: Power
generation cost
at margin
during vehicle
travel
Wiper, power
steering etc
~ 11.8-17.8%
efficiency
$0.49 - $0.74
/kWh
$0.62 / kWh
Case 3:
Generation cost
at plug load
while the vehicle
is running
Laptops etc
~ 8.9-15.9%
efficiency
$0.55 - $0.98
/kWh
$0.77 / kWh
Starter motor
etc
~ 8.3-16.3%
efficiency
$0.54 - $1.05
/kWh
$0.80 / kWh
Laptops etc
~ 6.2-14.6%
efficiency
$0.60 - $1.41
/kWh
$1.01 / kWh
Cases
Case 4: Power
usage through
stored battery
power
Case 5: Power
usage at plug
loads through
stored battery
power
Example
Efficiency
Table 4 shows generation efficiency and costs for five different cases. It shows
that, for a gasoline cost of $3.20 per gallon, the cost of power in a vehicle ranges from
about 37¢ per kWh (mechanical) to run the air conditioner, to about $1 per kWh
(electrical) to run appliances while the vehicle is turned off. We estimate that the average
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cost of generating electricity is at least 80 cents/kWh. This is about seven times the price
of residential electricity in the United States.
Conclusions
Light-duty vehicles increasingly provide many of the services normally associated
with buildings: heating, cooling, and ventilation, lights, entertainment systems,
communication systems, and even some kitchen services. All of these services consume
energy. Despite increasing emphasis on energy efficiency in buildings and in
automobiles, these uses of energy have received little attention: they are not included in
the calculations of automobile fuel economy as measured by the US EPA and used in the
corporate average fuel economy (CAFE) standards, nor are they included in the US EPA
Energy Star program.
Air conditioners in vehicles use an estimated 0.94 quads of primary energy
annually in the US, whereas air conditioners in residential US buildings use 2.3 quads,
even though vehicles are much smaller than residential buildings and even though people
spend only about an hour and a half per day in vehicles as opposed to 16 hours per day in
residences. Turler et al. (2003) have shown that thermal insulation and window
technologies can reduce heating and cooling loads, resulting in greater fuel efficiency for
conventional and hybrid vehicles, increased range for electric vehicles, increased
passenger comfort and reduced degradation of interior surface. Zhai (2001) suggests that
improved compressors could significantly improve the efficiency of automotive air
conditioning systems.
In comparison with automotive air conditioners, other appliances used in vehicles
have a relatively modest energy footprint, consuming about an order of magnitude less
energy than automobile air conditioners. However, this is offset by the high cost of
vehicle-generated electricity. Standby power for some of these appliances can have a far
greater impact than in stationary buildings, because stand-by power can drain the battery.
Moreover, there is a second kind of standby power associated with appliances in cars: all
of the accessories add weight to the vehicle and result in increased fuel use according to
their weight. For example, a DVD player and display, if kept in the vehicle continuously,
will use about as much petroleum to be transported in the vehicle as it will use in
electricity to operate.
Automotive heating systems, in contrast, are a model of efficient building heating
systems, in effect a mobile combined heat-and-power system
References
All-battery.com , 2008. www.allbattery.com/index.asp?PageAction=VIEWPROD&ProdID=451
Al-eds. 2008. http://www.al-eds.com/product.php?productid=16265&cat=280&page=1
Bosch, Robert. 1996. Automotive Handbook, 4th ed. Bentley Publishers, p. 813.
9
Crutchfield, 2007. Car Stereos Frequently Asked Questions.
http://www.crutchfieldadvisor.com/ISEOrgbtcspd/learningcenter/car/car_stereo/faq.html#8
Davis, Stacy, and Susan Diegel. 2007. Transportation Energy Data Book. Washington:
U.S. Department of Energy.
[DOE] Department of Energy. 2006. 2006 Buildings Energy Data Book. Washington,
D.C.: Building Technologies Program.
[DOE and EPA] Department of Energy and Environmental Protection Agency. 2008.
Advanced Technologies and Energy Efficiency.
www.fueleconomy.gov/feg/atv.shtml
[DOE] Department of Energy. 1995. Primer on Lead-Acid Storage Batteries, FSC-6910
hss.energy.gov/NuclearSafety/techstds/standard/hdbk1084/hdbk1084.pdf
Feneske, George, Robert Erck, Layo Ajayi, Ali Erdemir, Osman Eryilmaz, 2006.
Parasitic Energy Loss Mechanisms Impact on Vehicle System Efficiency, Project
15171, Argonne National Laboratory.
Fischer, S.K., J.R. Sand. 2007. “Total Environmental Warming Impact (Tewi)
Calculations for Alternative Automotive Air-Conditioning Systems.” SAE
970526.
Gado, Amr El-Sayed Alaa El-Din. 2006. Development of a Dynamic Test Facility for
Environmental Control Systems, Departmet of Mechanical Engineering,
University of Maryland, College Park.
Gates Corporation, Selecting the Right Drive System – costs and Performance.
www.gates.com/brochure.cfm?brochure=5262&location_id=6196
Hendricks, Terry. 2003. Multi-Variable Optimization of Electrically-Driven Vehicle Air
Conditioning Systems Using Transient Performance Analysis Paper read at SAE
Vehicle Thermal Management Systems.
[IEA] International Energy Agency, 2006. Light's Labour's Lost. Paris: International
Energy Agency. www.iea.org/Textbase/npsum/Light2006SUM.pdf
JC Whitney www.jcwhitney.com/media/jcwhitney/jcw/Documents/SZC01_11671G.html
Johnson, Valerie, 2002. Fuel Used for Vehicle Air Conditioning: A State by State
Thermal Comfort Approach. SAE 2002-01-1957.
10
Klepeis, N.E. et al. The National Human Activity Pattern Survey (NHAPS): a resource
for assessing exposure to environmental pollutants. J. Exposure Analysis and
Environmental Epidemiology 11,231-252, 2001.
Labkorea.com, 2008. www.labkorea.com/inverter.html
Leech, J., W.C. Nelson, R.T. Burnett, S. Aaron, M.E. Raizenne. 2002. “It’s about time:
A comparison of Canadian and American time-activity patterns.” Journal of
Exposure Analysis and Environmental Epidemiology, 12:427-432.
Levine, C., T. Younglove, M. Barth. 2000. “Interaction of Temperature, Humidity,
Driver Preferences, and Refrigerant Type on Air Conditioning Compressor
Useage.” Journal of the Air & Waste Management Association, 50:10, October.
Meissner, Eberhard and Gerolf Richter. 2003. Battery Monitoring and Electrical Energy
Management: Precondition for Future Vehicle Electric Power Systems. J Power
Systems 116, 79-98.
Meissner, Eberhard, and Gerolf Richter. 2005. The challenge to the automotive battery
industry: the battery has to become an increasingly integrated component within
the vehicle electric power system. Journal of Power Sources 144 (2): 438-460.
Mikhaylova, Zhanna, 2003. Power of a Car Stereo. The Physics Fact Book.
http://hypertextbook.com/facts/2003/ZhannaMikhaylova.shtml
Murphy, P. 2001. Spotlight on Automotive Interior Lighting. Machine Design.
http://machinedesign.com/ContentItem/70928/Spotlightonautomotiveinteriorlighti
ng.aspx
[NESCCAF] Northeast States Center for a Clean Air Future. 2004. Reducing
Greenhouse Gas Emissions from Light-Duty Motor Vehicles. Table D-2.
September
Polzin, S., X. Chu, L. Toole-Holt. 2004. “The Case for More Moderate Growth in VMT:
a Critical Juncture in US Travel Behavior Trends.” Slide 22. Presentation for US
Department of Transportation, November 3.
PPL, 2008. Car Refrigerator. http://www.pplmotorhomes.com/parts/rv-refrigerators/carrefrigerator.htm
Rugh, John, Lawrence Chaney, Jason Lustbader, John Meyer, Mukesh Rustagi, Kurt
Olson, and Rupert Kogler. 2007. Reduction in Vehicle Temperatures and Fuel
Use from Cabin Ventilation, Solar-Reflective Paint, and New Solar-Reflective
Glazing. Paper read at 2007 SAE World Congress, April 16-19, at Detroit.
11
Rugh, John, Valerie Hovland, and Stephen O. Andersen. 2004. Significant Fuel Savings
and Emission Reductions by Improving Vehicle Air Conditioners. 15th Annual
Earth Technologies Forum and Mobile Air Conditioning Summit, April 15, at
Washington.
Smarthome, 2008. Comfortable 12-Volt Car Seat Cover Heats to Adjustable Levels
http://www.smarthome.com/92901b.html?src=WG1003498
SterlingTek, 2008. 150 Watt Power Inverter Car Adapter
http://sterlingtek.com/caradforalll.html
Turler, Daniel, Hopkins, Deborah, and Goudey, Howdy. Reducing Vehicle Auxiliary
Loads Using Advanced Thermal Insulation and Window Technologies. Advances
in Automotive Climate Control Technologies, 2003. SAE World Congress.
Z28 Forum, 2007. Car Audio-Wattage Explained.
http://www.z28.com/forum/showthread.php?t=36069
Zhai, Kelvin. Could Automotive HVAC Become an EPA/DOE Energy Star? Future
Transportation Technology Conference & Exposition, August 2001, Costa Mesa,
CA, USA, Session: Energy, Environment and Economics.